This invention relates in general to an induction heating arrangement for metal bar stock and more particularly to a heating arrangement for, and method of heating, an end length of forging bar stock to a uniform temperature throughout directly following the cut-off therefrom of a previously heated and forged end length.
In the commercial production of forgings from metal bar stock, it is common practice to heat an end length portion of the extended bar stock to the desired forging temperature and then subject it to the required forging operations to form it into the desired forging which is then cut off the end of the bar. Thereupon, the end of the forging bar is again heated to forging temperature throughout a similar end length portion which is then formed into the desired forging and the latter cut off the bar end. This procedure is repeated until all of the usable length of the bar stock has been utilized. This forging practice is known in the art as "heating off the end of the bar". After a completed forging is cut off the bar end, a residual hot end spot remains at the bar end.
Where the heating of the forging bar or workpiece is carried out in a combustion furnace, this residual hot spot at the forging bar end does not create any particular problem in heating the next bar end length portion to uniform forging temperature throughout its full extent since the furnace acts to bring the entire bar end length portion up to the same final temperature despite the existence of the hot end spot at the start of the heating operation. However, the residual hot spot at the bar end does create a problem where the heating of the bar end length portion is carried out in induction heating coil systems since these induce energy into and heat the workpiece as a function of the time the workpiece is being heated in the heating coil. Consequently, if the workpiece, at the time when induction heating thereof first begins, has two adjacent sections at substantially different temperatures, then after induction heating this temperature differential between the two workpiece sections will still exist. Accordingly, if a forging bar with a residual hot end spot is heated throughout an end length portion within an induction heating coil and the cool or as yet unheated section of the bar end length portion is brought up to forging temperature, the hot end spot will then be overheated and may even melt.
One proposal for overcoming this problem as disclosed in U.S. Pat. No. 4,075,450, Lavens, hereby made of record in the present application, has been to initially locate the residually heated end of the workpiece or forging bar entirely outside the full heating zone of the induction heating coil, while the cool or as yet unheated portion of the bar end length portion within the coil heating zone is brought up to substantially the same temperature as the residually heated bar end. Thereupon, the forging bar is retracted within the induction coil heating passageway, as by means of hydraulic cylinder operated pusher rod means, to locate the bar end entirely within the heating zone of the heating coil along with the rest of the bar end length portion to be forged, and the inductive heating of the forging bar then continued until the end length portion to be forged has attained the required forging temperature. This two-stage induction heating operation thus brings the entire bar end length portion to be forged to a uniform forging temperature despite the temperature variations that originally existed in such bar portion before the start of the heating operation. However, because it requires among other things the provision of a hydraulic cylinder operated push rod mechanism and a water cooling system for the push rod, the induction heating system of the aforesaid Lavens patent is of rather complicated and expensive character.